CVE-2020-8834: security - CVE-2020-8834: Linux kernel Power8 conflicting use of HSTATE_HOST_R1 vulnerability

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Date: Mon, 6 Apr 2020 09:01:12 -0700 From: Steve Beattie <steve.beattie@…onical.com> To: oss-security@…ts.openwall.com Subject: CVE-2020-8834: Linux kernel Power8 conflicting use of HSTATE_HOST_R1 vulnerability

Hello,

KVM in the Linux kernel on Power8 processors has a conflicting use of HSTATE_HOST_R1 to store r1 state in kvmppc_hv_entry plus in kvmppc_{save,restore}_tm, leading to a stack corruption. Because of this, an attacker with the ability run code in kernel space of a guest VM can cause the host kernel to panic.

There were two commits that, according to the reporter, introduced the vulnerability:

f024ee098476 (“KVM: PPC: Book3S HV: Pull out TM state save/restore into separate procedures”) 87a11bb6a7f7 (“KVM: PPC: Book3S HV: Work around XER[SO] bug in fake suspend mode”)

The former landed in 4.8, the latter in 4.17. This was fixed without realizing the impact in 4.18 with the following three commits, though it’s believed the first is the only strictly necessary commit:

6f597c6b63b6 ("KVM: PPC: Book3S PR: Add guest MSR parameter for kvmppc_save_tm()/kvmppc_restore_tm()") 7b0e827c6970 (“KVM: PPC: Book3S HV: Factor fake-suspend handling out of kvmppc_save/restore_tm”) 009c872a8bc4 (“KVM: PPC: Book3S PR: Move kvmppc_save_tm/kvmppc_restore_tm to separate file”)

Thus, the only upstream kernel affected is the 4.17 kernel – Ubuntu’s 4.15 kernels are affected because we backported 87a11bb6a7f7 to our 4.15 kernels. It does not appear to have been cherrypicked to any of the upstream stable kernels.

Discovered by Gustavo Romero and Paul Mackerras. Ref: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1867717 CVE: CVE-2020-8834

– Steve Beattie <sbeattie@…ntu.com> http://NxNW.org/~steve/

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